An oxide superconducting material comprising a single-crystal form REBa2Cu3O7-x (RE means rare earth element) phase in which a RE2BaCuO5 phase is dispersed has a high critical current density (below, also indicated as “Jc”), so is magnetized by cooling in a magnetic field or by pulse magnetization and can be used as a superconducting bulk magnet able to generate a strong magnetic field.
A superconducting bulk magnet has the excellent feature of being able to generate an extremely powerful magnetic field in a compact space, but since an extremely strong magnetic field is sealed in the compact space, a large electromagnetic stress acts inside an oxide superconducting bulk member. This electromagnetic stress acts so that the sealed-in magnetic field spreads, so is also called “hoop stress”. In the case of a 5 to 10 T class strong magnetic field, the electromagnetic stress which acts sometimes exceeds the mechanical strength of the material of the superconducting bulk member itself. As a result, the oxide superconducting bulk member is liable to break. If the oxide superconducting bulk member breaks, the superconducting bulk member can no longer generate a strong magnetic field.
If possible to prevent breakage of a superconducting bulk member by electromagnetic stress, the features of a superconducting bulk magnet of compactness and a strong magnetic field can be expected to be made use of for help in improving the performance of the equipment and reducing the size and lightening the weight of equipment in applications utilizing magnets such as chemical transport systems utilizing small sized NMR (nuclear magnetic resonance) magnetic members or magnetic force.
To prevent breakage of a superconducting bulk member by electromagnetic stress, for example, PLT 1 proposes a superconducting bulk magnet configured by a circular columnar-shaped superconducting bulk member and a metal ring surrounding the same. By configuring the magnet in this way, at the time of cooling, a compressive stress due to the metal ring is applied to the superconducting bulk member. This compressive stress has the effect of reducing the electromagnetic stress, so it is possible to suppress breakage of the superconducting bulk member. In this way, PLT1 shows that it is possible to prevent breakage of a circular columnar-shaped superconducting bulk member.
In this regard, to use a general size (for example, diameter 40 to 100 mm or so) single-crystal form oxide superconducting material to generate a high strength magnetic field by magnetization, it is effective to make the single-crystal form oxide superconducting materials ring shaped and generate strong magnetic fields inside the ring. At this time, it is further effective to stack these rings with their inner circumferential and outer circumferential axes aligned.
In general, by working disk-shaped bulk materials into ring shapes, it is possible to utilize the relatively high strength, uniform magnetic fields at the insides of the rings. Due to this, application for NMR or MRI (magnetic resonance imaging) etc. where particularly high uniformity is demanded becomes possible.
Further, PLT 2 discloses a superconducting magnetic field generating device which is manufactured by combining seven hexagonal-shaped superconducting bulk members, arranging a reinforcing member comprising a fiber reinforced resin etc. around them, and further arranging a support member comprising stainless steel, aluminum, or other metal at an outer circumference thereof.
PLT 3 discloses an oxide superconducting bulk magnet obtained by stacking ring-shaped bulk superconducting members with thicknesses in the c-axial direction of the crystal axes of 0.3 to 15 mm.
Further, PLT 4 discloses a superconducting bulk magnet obtained by stacking a plurality of ring-shaped superconducting members reinforced at the outer circumferences and inner circumferences.
Furthermore, PLT 5 discloses a superconducting bulk magnet obtained by stacking superconducting members having multi-layer ring structures in the diametrical direction.
Further, PLT 6 discloses a bulk magnet comprised of a single bulk member reinforced at its outer circumference and top and bottom surfaces. PLT 7 discloses a bulk magnet having high temperature superconducting members placed inside a cup-shaped conductive member and having conductive members sandwiched between the plurality of high temperature superconducting members. However, in FIG. 3 of PLT 7, while the conductive members 17b and high temperature superconducting members contact each other and transfer heat, the concept of reinforcement of the superconducting bulk members against the electromagnetic force is not shown.